1. Field of the Invention
The present invention concerns a new mode configuration of laser cavity resonator for producing a single frequency output from lasers.
Laser oscillators employing active media with inhomogeneously broadened gain profiles are inherently multimode devices. If .DELTA..nu..sub.D is the linewidth of the active medium and L the mirror separation of the laser Fabry-Perot interferometer, then the number of discrete oscillation frequencies in a single transverse mode (i.e. longitudinal modes) is approximately given by ##EQU1## in which c is the speed of light. For the 6328 A He-Ne gas laser, L is typically 1.5 m and .DELTA..nu..sub.D .apprxeq.1600 Mc/s, giving N.apprxeq.15. Unless special techniques are employed to produce a phase-locked laser, each of the oscillating longitudinal modes will have fixed amplitude and random phase. The lack of phase correlation between modes will result in random intensity fluctuations in the output causing the laser to have excess photon noise. The desirability of having a single-frequency laser oscillator is evident then, both from the point of view of noise and from the point of view of eventual usefulness in communications systems. Such an oscillator would find application, for example, as a carrier source in a communications channel, or as a local oscillator in an optical heterodyne detector, as well as an ultra-narrow linewidth source for certain spectroscopic studies.
2. Description of the Prior Art
A number of passive mode filtering techniques are available for producing a single mode (i.e. a single frequency single transverse mode) oscillator.
In the article "Characteristics of a Single-Frequency Michelson-Type He-Ne Gas Laser" published in IEEE Journal of Quantum Electronics, Vol. QE-2, No. 8, August 1966, M. DIDOMENICO disclosed a single mode arrangement in the form of a modified Michelson interferometer. A schematic diagram of this Michelson type laser is shown in FIG. 1, from which it can be seen that the basic modification is the introduction of a common feedback mirror M.sub.c for coupling the two branches of the Michelson at the beam splitter BS. In the configuration shown, two long gain paths are used with small path length differences .DELTA.L=L.sub.1 -L.sub.2. The path length difference .DELTA.L is responsible for the mode suppression properties of the device, and the two long gain paths allow one to achieve high single-mode power. The coincidences between resonances of the cavity formed by mirrors M.sub.1 and M.sub.c and those of the cavity formed by mirrors M.sub.2, BS and M.sub.c are found to be separated in frequency by c/2.DELTA.L. These coincident frequencies are the preferred resonances of the overall coupled system and correspond to the condition where zero energy is coupled out of the beam splitter. Mode discrimination at all other noncoincident frequencies comes about because energy is coupled out of the beam splitter with the result that the system becomes lossy at these frequencies.
The article of S. LIBERMAN and J. PINARD "Single Mode CW Dye Laser with Large Frequency Range Tunability" published in Applied Physics Letters, Vol. 24, No. 3, February 1974 disclosed a dye laser in which a Michelson interferometer is substituted for one of the mirrors of a Fabry-Perot interferometer.
In the lasers of these references, the separation between coincidences is found to be EQU .DELTA..nu.=c/2.DELTA.L
instead of .DELTA..nu.=c/2L for the separation between discrete adjacent oscillation frequencies in a single transverse mode laser oscillator. The passive transmission loss out of the beam splitter T.sub.q at a noncoincident frequency displaced from a coincident frequency by qc/2L (q integer and L=(L.sub.1 +L.sub.2)/2 is (for 3-dB beam splitter): ##EQU2##
The term in the denominator is due to the fact that the laser active material is located along the two split branches of the Michelson interferometer and not in the common branch. If it were located in the common branch, the passive transmission loss out of the beam splitter would be EQU T.sub.q =4R.sup.2 sin.sup.2 .pi.q(.DELTA.L/L) (1)
where R is the reflectivity coefficient of the beam splitter (R=1/2 for a 3-dB beam splitter).
The coincident resonances take place for q=0, L/.DELTA.L, 2L/.DELTA.L, . . . . If .DELTA.L/L is small (FIG. 2), the period between two adjacent coincidence resonances is large but the noncoincident adjacent modes are poorly attenuated. If .DELTA.L/L is large (FIG. 3), the period between two adjacent coincidence resonances is small but the noncoincident adjacent modes are significantly attenuated.
In both cases, an interferometer of this nature requires an adjustment of the quantity .DELTA.L having an accuracy and a stability exceeding that possible to achieve by mechanical means (as a result of thermal drift). This is why, as described in the above-mentioned reference to L. LIBERMAN et al., one of the Michelson interferometer mirrors is mounted on a piezoelectric shim, as well as, moreover, the laser's main mirror; these shims receive electric voltage from an automatic control device using an error signal obtained by comparison with a reference voltage.
Nonetheless, it can be difficult to render a laser single mode due to the only fact that a Michelson interferometer replaces one of its end reflectors. This is particularly true as regards a dye laser, i.e. a laser whose amplification medium is made up of organic molecules in solution, the fluorescence of which, when excited by another laser, is omitted over a very wide range typically of several hundred Angstroms in wavelength. Experience has shown in actual fact that a dye laser equipped with an ordinary Michelson interferometer can only be made single mode at low powers.
U.S. Pat. No. 3,495,911 issued Feb. 17, 1970 to Robert D. DURAND et al., described an extended range interferometer illuminated by a multimode light source. Considering that multimode light sources suffer the disadvantage, for measuring distances with fringe counting interferometers, in that there is a periodic minimum in contrast and in discernability of the fringes as the distance increases over which the fringes are counted, it is proposed to illuminate two Michelson interferometers by the same multimode light source each interferometer having a reference path and a variable path and to direct the output beam of the two interferometers in the same direction towards two different photoreceivers. By properly positioning the paths associated with the two photoreceivers, the fringe visibility at one detector can be substantially at a maximum while the fringe visibility at the other detector can be at a minimum.
In the Durand's double interferometer, the two Michelson interferometers are illuminated by a multimode laser but do not form the cavity resonator of a single mode laser.